Conventional optoelectronic devices (for example, electro-absorption modulators or EAMs) comprise a waveguide on a base that is generally a semiconductor substrate such as a silicon substrate. Waveguides built upon this base comprise three layers: a core layer, a bottom cladding layer, and an upper cladding layer; which are configured to guide a light signal through the core layer by total internal reflection. The core layer is a light-transmitting medium, which is conventionally a thin (relative to an active material discussed below) silicon layer located on top of an insulating layer such as a buried oxide or BOX layer. The BOX layer, as part of the waveguide (bottom cladding) is located on top of the silicon substrate and functions to confine the light into the light-transmitting medium. An optoelectronic device may comprise a waveguide with an optically active region (also referred to as an active waveguide), for example an electro-absorption medium, deposited in a cavity in the silicon layer (i.e. atop the BOX layer). Typically, a thin silicon layer is left on the bottom of the cavity between the BOX layer and the optically active region as a crystal seed for the active material to be grown epitaxially. Both the silicon seed layer and the BOX layer may function as the bottom cladding for the active waveguide. Usually, the epitaxial growth for the active material needs a further active material seed layer located upon the silicon seed layer in order to obtain a high quality crystal structure of the desired active region. For example, a seed layer of germanium may be grown when an active layer of silicon-germanium is to be grown. The uniform and continuous silicon layer must be kept relatively thin so as to maintain the coupling efficiency between a passive waveguide (e.g. a non-optically active waveguide) and the active waveguide at a useful level. In previous optoelectronic devices, the thickness of the silicon layer is around 0.2 μm.
To fabricate a known EAM as described above from a silicon wafer, the silicon layer above the BOX must be etched to a thickness of around 0.2 μm from an initial thickness of around 3 μm. It is difficult to do this consistently, and so problems with yield may arise.
Known optoelectronic devices which operate at 1310 nm wavelengths suffer from a number of issues. For example, in Mach-Zehnder interferometer based devices operating at this wavelength have a very large footprint on a photonic circuit, which can result in a very large parasitic capacitance. Moreover, the driver circuit for such a device is very complicated and will often require a distributed electrode and transmission line design. Quantum-confined Stark effect devices operating at this wavelength show a high polarization dependency (which means that the performance difference such as insertion loss and extinction ratio for TE and TM modes can be out of the acceptable range) as well as a high sensitivity to manufacturing process tolerance. Operational bandwidth is also limited in a trade-off with extinction ratio i.e. for a given extinction ratio the maximum bandwidth is correspondingly limited.
Until now, it has been accepted that a silicon seed layer and a BOX layer are necessary beneath the optically active region as bottom cladding in order to make the optoelectronic device function. However, the inventors have realised that the silicon seed layer and the BOX layer are not necessary and can be replaced by other material that has a crystal structure with a lower refractive index than that of the optically active region.